ABS LIGHT STAYS ON

His ABS light has been on for the past week. The light comes on when he starts the suv and stays on. However, he notices no difference with his braking performance.

We are taking the car into the dealers next week to have it looked at. Unfortunatly, are car is not covered by warranty anymore. So I would like to have some vague idea about what the problem might be and how extremely horrible the $$ damage is gonna be.
TIA!